1. Field of the Invention
This invention relates to endpoint detection during dry etching processes such as plasma assisted discharge processing operations. More particularly, the present invention relates to a method and apparatus for such dry etching endpoint detection.
2. Description of the Related Art
A number of etching methods have been utilized in semiconductor device fabrication processes, employing plasma discharges and charged particles produced by such discharges. These etching methods by plasma or dry etch methods have been used more and more widely in device fabrication.
During device fabrication processes, a device portion to be etched typically comprises a layer of a first material disposed on top of a layer of a second different material, with certain portions of the first material exposed to plasma through a mask. The device portion is then subjected to plasma etching. The aim of the plasma etching process is to etch away portions of the first material until the second material is exposed, without etching any of the second material. Therefore, it is critical to determine when the plasma etching process should be terminated.
One of the known methods for monitoring the etching of these layers is to monitor the intensity of the light emission from the plasma discharge. Many processes have particular spectral lines or regions which originate from some chemical constituents in the plasma such as reactant species or plasma etch products. By monitoring the intensity at the wavelength characteristic of these species in the plasma, the concentration of those species in the plasma can be determined, which is directly related to the etching processes.
For example, there is disclosed a method in Japanese Laid-Open Patent Application No. 59-94423, in which (1) the intensity of light emission from certain reaction products is monitored, (2) the second derivatives of the intensity is then calculated with time and (3) an endpoint is determined when the value of the second derivative exceeds a predetermined threshold level.
In this method, however, there have been found several shortcomings such as, for example, fluctuations with time in the light emission intensity which is extracted from a reaction etching chamber, thereby resulting in noises included in the intensity signals. These noises originate from several sources depending, for example, on the method used to generate the plasma and the kind of material to be etched.
When the light emission to be extracted is passing through the material to be etched, emitted light is affected by interference depending on the wavelength of the light thereby resulting in noise occurring in the outputted intensity signals, which will be detailed hereinbelow.
FIG. 3 is a schematic block diagram, illustrating major portions of a known plasma etching system. The etching system includes a reaction chamber 1, a body 16 placed therein to be etch treated, a suitable evacuating apparatus (not shown) attached thereto through a connection 2, and a gas feeding system (not shown) also attached thereto through a connection 3 to supply gaseous materials appropriate for the processing. The body 16 is disposed generally on top of a substrate 16a.
Also provided within the reaction chamber 1 are lower and upper electrodes 4 and 5, having broad faces. These broad faces of the electrodes are placed spaced apart horizontally to form a parallel plate reactor. The lower electrode 4 is supplied with power, generally in the radio-frequency (RF) range, by a power source 6 and serves as a support for the body 16 to be treated. The upper electrode 5 is maintained at ground potential.
Still further, the reaction chamber 1 includes a window 9 provided on a side wall portion thereof, for extracting the light emission which is produced within the chamber 1 by the interaction between the plasma and the body 16 being treated. A fiber optic cable 10 links the window 9 to a spectrometer 11, where the intensity of the extracted light emission at appropriate wavelengths is obtained and transformed further to signal outputs according to the detected intensity.
A shown in FIG. 3, the window 9 has been typically located on the side wall portion of the reaction chamber 1 above the horizontal plane which is defined by the surface of the body 16, and on the same side of the chamber 1 at which plasma processing is taking place. The light emission extracted from the window 9 may therefore include several components such as (1) a component I.sub.0 which originates from the plasma region 8 between the electrodes, (2) another component I.sub.R which originates from the reflection by the inner walls of the reaction chamber 1 and (3) still another component I.sub..theta. caused by the reflection from the body 16 being treated, which is assumed to be transparent in the spectral wavelength of the measurements.
Since the component I.sub..theta. results from the interference of (1) light reflected by the surface of the body 16 and (2) light reflected by the interface between the body 16 and the underlying substrate 16a, as illustrated in FIG. 4, the component I.sub..theta. reflected by the body 16 has an intensity variation with peak intensities under the condition EQU 2h.multidot.(n.sub.2.sup.2 -n.sub.1.sup.2.multidot.sin.sup.2 .theta.).+-..lambda..sub.0 /2=m.multidot..lambda..sub.0 (1)
(m=1,2,3, . . . ) PA1 with h being a thickness of the body 16, PA1 n.sub.1 a refractive index of the reaction chamber ambience, PA1 n.sub.2 a refractive index of the body 16, PA1 .theta. an angle between a reflected light beam and the normal of the surface of the body 16, and PA1 .lambda..sub.0 a wavelength of light to be detected.
The value h in the above equation (1) decreases as the etching process proceeds, thereby resulting in a periodic change in the emission intensity over time. Accordingly, the amount of change in thickness h.sub.0, corresponding to one intensity peak to the next in the I.sub..theta. versus time curve, is expressed by EQU h.sub.0 =.lambda..sub.0 /{2(n.sub.2.sup.2 -n.sub.1.sup.2.multidot.sin.sup.2 .theta.)} (2).
This results in the period .tau. for the aforementioned periodic change as EQU .tau.=h.sub.0 /R (3)
in which R is an etching rate.
As an exemplary illustration, an etching process is assumed in a case where a body 16 composed of a polysilicon layer is etch treated under the following conditions; n.sub.1 1.0, n.sub.2 5.05, .theta. 60.degree. and .lambda..sub.0 426 nanometers. With an etch rate of 300 nanometers/second, .tau. is found to be 8.56 seconds.
The aforementioned periodic change in intensity gives rise to concomitant changes in the second derivative values of the light emission intensity which is utilized for determining the endpoints in the aforementioned example. When these changes exceed a predetermined value, this may result in erroneous endpoint detection.
Several methods have been disclosed to obviate the above-mentioned difficulties, such as, for example, by eliminating the periodic changes or fluctuations. For example, Japanese Laid-Open Patent Application No. 1-226154 discloses several methods, in which an additional "external" means such as a low-pass filter, phase adjusting unit or input signal controller is additionally incorporated in a signal processing circuitry to smooth the fluctuation with time, to thereby eliminate the above periodic changes.
By "external" is meant that this process attempts to eliminate the change after the extraction of light emission or in the course of the signal processing, instead of eliminating it before the extraction or during the collection of light emission inside the chamber. With these methods, the aforementioned difficulties may be obviated to some extent.
Referring to FIG. 5, the above-mentioned method will be exemplified hereinbelow for the case of a plasma etching system with signal processing units incorporating a low-pass filter 20.
The plasma etching system including the reaction chamber is illustrated and described above with respect to FIG. 3. Accordingly, the various portions of FIG. 5 which are common to the system of FIG. 3 are designated with similar numerals and are described only briefly when relevant.
In contrast to the plasma etching system of FIG. 3, the etching system of FIG. 5 is further provided with a dipole magnet 18 which is located spaced apart above the upper electrode 5 and connected to a rotation controlling motor 19 via a rotation axis through the top plate of the reaction chamber 1, so as to rotate the magnet 18 eccentrically around the rotation axis.
A window 9 is provided on a side wall portion of the reaction chamber 1 for extracting the plasma emission which is produced within the chamber 1, originating from chemical constituents in the plasma such as reactant species or plasma etch products. A fiber optic cable 10 links the window 9 to a spectrometer 11 and appropriate wavelengths are selected by a spectrometer 11. The intensity of the extracted light emission at the thus selected wavelengths is transformed to electrical signal outputs by a photomultiplier tube 12, amplified by an amplifier 13, then fed to a low pass filter 20, thereby resulting in amplified intensity signals.
The amplified intensity signals are then fed, through an offsetting adder 21 to a gain amplifier 22, through an analogue-to-digital (A/D) converter 14 and inputted to a microcomputer 15 to be discriminated with respect to a predetermined threshold level.
The microcomputer 15 is configured to be capable of (1) adding offset values to the offsetting adder 21 through the D/A converter 23, (2) adjusting gains of the gain amplifier 22 and (3) turning on or off a power source 6 for the etching system.
An appropriate algorithm is stored in the microcomputer 15, which is capable of detecting an endpoint of the plasma etch processing with respect to a predetermined threshold value. Endpoint detection can be achieved in such a manner, for example, by calculating second derivatives of the emission intensity versus time curve, based on the data outputted from the low pass filter 20, and then comparing the thus obtained value to the predetermined value. Upon detecting an endpoint, the power supplied by power supply 6 for the etching system is terminated by instruction from the microcomputer 15.
With the present construction of the above-mentioned endpoint detection system, the intensity of the light emission changes with the rotation of the dipole magnet 18, and similar changes result in the intensity signals amplified through the photo-multiplier tube 12 and amplifier 13. The change is typically observed as a large gradual change over a relatively long period of time accompanied by smaller changes with a shorter period, caused by the rotation of the magnet 18, as illustrated in FIG. 6A. Namely, the resulting signals generally include noises (or intensity fluctuations with time) with relatively short periods of time.
When the above-mentioned fluctuated signals are fed thorough the low-pass filter 20, the noises with the short period are smoothed to thereby result in a gradual change such as illustrated in FIG. 6B. Therefore, the endpoint detection becomes feasible with more ease with this method.
When feeding the outputted signals through the low pass filter 20 as above, a time delay generally results in the signals being inputted to the microcomputer 15. This is illustrated in FIG. 6B, in which the change of the emission intensity curve without the time delay is shown as a dotted solid curved line, while the delayed signal change obtained after feeding the signal through the low pass filter 20 is shown as a solid curved line.
In Japanese Laid-open Patent Application '154, there is disclosed another method which additionally incorporates a phase adjusting unit in the signal processing circuitry, as illustrated in FIG. 7.
Referring to FIG. 7, the above-mentioned method will be exemplified hereinbelow, in which the etching system with the signal processing units was already illustrated in FIGS. 3 and 5. Various portions of FIGS. 3 and 5 as described above, are common to the detection system of FIG. 7. Accordingly, the common elements are designated with similar numerals and described only briefly, when relevant.
In contrast to the endpoint detection system of FIG. 5, the plasma processing system of FIG. 7 is provided with a phase adjusting unit 30, in place of the low pass filter 20 of FIG. 5.
When intensity signals are fed into the phase adjusting unit 30, having such fluctuations as shown in FIG. 6A, the intensity signals are processed in the phase adjusting unit 30, such that (1) intensity signals are formed so as to have a certain time delay, or phase shifted, with respect to inputted (or non-phase-shifted) signals and (2) the phase shifted signals are added to the non-phase-shifted signals, to thereby form averaged signals to be outputted. For example, this signal averaging is carried out by (1) forming two sets of signals, each set being phase-shifted by 2.pi./3 and 2.times.2.pi./3, respectively, and (2) adding the two sets of signals together with the original non-phase-shifted signals, thereby forming resultant signals.
The timing of the signal addition in the phase adjusting unit 30 is performed synchronously with timing signals from a rotation controller 31, which controls the rotation of the motor 19.
With the present construction of the signal processing units, the above-mentioned fluctuated outputted signals can be smoothed by feeding them into the phase adjusting unit 30, thereby averaging the plurality of sets of intensity signals. The endpoint detection therefore becomes feasible with more ease with this method.
However, feeding the outputted signals through the phase adjusting unit 30, generally results in a certain time delay to the averaged signals caused by the phase shifting process.
In Japanese Laid-open Patent Application '154, there is disclosed still another method which incorporates an input signal controller 40 in the signal processing circuitry, as illustrated in FIG. 8.
Referring to FIG. 8, the above-mentioned method will be exemplified hereinbelow, in which the plasma etching system with signal processing units was already illustrated in FIGS. 3 and 5. Various portions of FIGS. 3 and 5 are common to the system of FIG. 8 and are designated with similar numerals and described only briefly, when relevant.
In contrast to the plasma etching system of FIG. 5, the system of the FIG. 8 is provided with an input signal controller 40 in place of the low pass filter 20 of FIG. 5.
When intensity signals are fed into the input signal controller 40, having such fluctuations as shown in FIG. 6A, the input signal controller 40 carries out sampling according to the period of the fluctuation.
The timing of the signal sampling in an input signal controller 40 is adjusted synchronously with timing signals from input timer 41, which also controls the rotation of the motor 19.
With this construction of signal processing units, the thus obtained signals are considered approximately the same as those intermittently inputted from non-fluctuated signals, and the above-mentioned fluctuated outputted signals from the amplifier 13, can be smoothed by feeding them into the input signal controller 40.
Because of the sampling operation, however, outputted signals are obtained only at a time interval which is equal to the period of the rotation of the motor 19. Therefore, the accuracy of the endpoint detection is affected by the time interval which is inherently included in the sampling method itself.
In these three methods disclosed in Japanese Laid-open Patent Application '154, a motor 19 is used and signal fluctuations result from the rotation of the motor 19. The period of the fluctuation is therefore equal to that of the motor rotation. In addition, since the aforementioned effect of the light interference within, or on the body to be etched, is also dependent upon the rotation, the resultant fluctuation of the collected light intensity therefore has the same period as that of the motor rotation. Therefore, these three methods are also useful to eliminate the interference of emitted light beams.
As described hereinabove, the prior methods of dry etch endpoint detection are useful to a certain extent for their capability of eliminating the noises (or intensity fluctuations with time) of the emission intensity. However, these methods are not completely satisfactory because of a certain time delay induced in the averaged signals caused by the phase shifting process, or the reduced accuracy affected by the time interval associated with the sampling method.
It would therefore be desirable to provide a method and an apparatus for eliminating these errors in the dry etching endpoint detection.